1. Field Of The Invention
This invention relates to a damper for an air handling system. More particularly, this invention relates to a damper of the parallel flow type which has been modified to provide it with pressure drop and air flow characteristics more closely corresponding to those of an opposed blade damper.
2. Description Of The Prior Art
Air handling systems for industrial and commercial applications utilize dampers to control the flow of air through the system. Each such damper has a plurality of blades whose longitudinal axes are parallel to one another, and the position of each blade in the damper is adjustable about its longitudinal axis between an open position in which the blade extends generally parallel to the flow of air through the system and a closed position which the blade extends transversely to the direction of flow of air through the system. Multiple blade dampers generally fall into one or another of two basic types, a parallel blade damper in which each of the blades of the damper rotates about its longitudinal axis in the same direction in the adjustment of the positions of the blades, and an opposed blade system in which, in the adjustment of the positions of the blades, each blade rotates in an opposite direction with respect to the blades on the opposite sides thereof. FIG. 2 of U.S. Pat. No. 3,267,962 (L. Josephson et al.) illustrates an opposed blade type of damper and FIG. 12 of the same reference illustrates a parallel blade damper.
Parallel blade dampers and opposed blade dampers each have certain flow characteristics which are desirable for certain applications and which are less desirable for certain other applications. For example, in an air handling system with a pair of opposed blade dampers, one of which is operated in an opened position and the other of which is operated in a closed position, and vice versa, there will be a significant loss of total air volume in the system at the midpoint of the control range of each of the dampers, whereas with dual, parallel blade dampers, the air volume at the midpoint of the control range will be much higher.
In many air handling systems it is desirable to have a damper which operates partly with the air flow characteristics of an opposed blade damper and partly with the air flow characteristics of a parallel blade damper, especially in connection with the air flow rate at the midpoint of the control range. Further, in certain existing applications, due to changes in the requirements of the system, it is desirable to be able to modify the flow characteristics of an existing mechanically sound damper by means of a simple retrofit operation, without removing and replacing the damper. It is especially important to be able to accomplish these results in the case of a mixing damper installation, in which a pair of dampers are operated in tandem between operating conditions in which one damper is substantially fully open and the other one is substantially fully closed, and vice versa, for example, to control the relative amounts of fresh and return air flowing within the system and to provide a suitable mixing of the return air and the fresh air.